It is very desirable to reduce the distance between the central processor and the memory so that operand and instruction access times are reduced to a minimum. The central processor can have a faster cycle time when operating at cryogenic temperatures. When the memory is placed with the central processor within the cryo-cooled system, both the memory and the central processor not only benefit from the lower temperatures that they operate at but also by the reduced distance between the central processor and the memory. Due to the long lines associated with Dynamic Random Access Memory (DRAM) memories, a greater performance improvement can be anticipated for the DRAM memory than for the logic. When the memory is so close to the processor, it can negate the need for a Cache Memory and thus have a greater savings.
It is desirable to cool the memory and the central processor by means of a cold head of a cryo cooler. The cold head of a cryo cooler is the heat exchange point whereby a cryo cooling system picks up or absorbs the heat from electronic devices. The heat absorption is accomplished by the cooling of a plate which is in surface-to-surface contact with an external surface of a cooling module. The cooling module contains these electronic devices. The heat is conducted from the electronic devices to the mating surface of the cooling module. The mating surface of the cooling module, being in face-to-face relationship with the plate of the cold head, then transfers the heat generated by these electronic devices to the refrigeration system through the cold head, and the heat is conveyed to another point or station where this heat may be dissipated in a conventional manner.
The access time between the Cache Memory and the Main Memory is greatly reduced in this configuration due to the fact that the Main Memory is placed electrically close to the processors due to this unique configuration under cryogenic temperatures. It could be possible to realize a memory access speed improvement of up to a factor of 5 in this unique configuration.
By placing the central processor and the memory within the same cooling module or low temperature conduction module and as closely together as possible, the performance of the system may be increased by a factor of as much as 2.5.
With the requirements for large quantities of memory and very close placement of the memory to the central processor, the memory chips are combined into assemblies known as memory cubes. Memory cubes are a plurality of memory chips packaged in a generally cubical shape; the chips are stacked and bonded together to form the cube. The electrical contacts for each memory chip are typically routed to one edge of the chip so that the contacts may be exposed to the circuitry used to connect the memory to the central processor.
In memory chips stacked to form a cube, one very significant disadvantage is that heat dissipation is significantly curtailed; accordingly, the chips have a tendency to operate at very high temperatures. Memory cubes are extremely difficult to cool because the heat must be transmitted and conducted from the center of the cube to the exterior surfaces. The silicon slabs on which the chips are made are poor heat conductors at room temperatures; therefore, the heat is prone to build up in the chips and in the memory cube during any operation. But at cryogenic temperatures, the silicon slabs are good heat conductors; therefore allowing the heat to be conducted away.
In a vacuum, the chips and memory cube must rely on conduction for cooling. Only surfaces which are engaged by a conduction member can be cooled, further complicating cooling of memory cubes in vacuums.
Attempts to operate memory cubes at room temperature have proven to be unsatisfactory because of the heat build up within the cube structure and due to the stresses on the materials of the memory chip, created by such heat build up. In many cases, the chips will fail when the centermost portion of the cube attempts to expand much more than the periphery of the cube, thereby breaking electrical paths or cracking the silicon substrates, thereby causing chip failure.
A sophisticated cooling system is required to make memory cubes functional. Low temperature offers the best opportunity to achieve this goal with enhanced performance. Memory cubes have the distinct advantage of producing very short cycle times and reduction of system complexity, for example the elimination of Cache Memory. Therefore, some short cycle systems are dependent upon formation of memory chips into cubes in order to reduce the interconnection distances between individual chips. Dense placement of memory chips permits the placement of a multitude of chips close to the computer processor.
If adequate cooling of the memory cubes is possible, an arrangement to position a significant quantity of memory, in the form of memory cubes, must be devised in close proximity to the computer processor. This requires the maintenance of both the computer processor and the memory cubes at very low temperatures in order to prevent overheating and to enhance the cycle times of the memory and processor. Whenever the DRAM is maintained in an environment of cryogenic temperatures, the added benefit of reduction in the number of cycles and the electrical requirements necessary to refresh the DRAMs result in an added benefit to the system designer, inasmuch as this further reduces overheating or makes the memory and processor available for their primary functions.
Prior art attempts to cool electronic elements to cryogenic temperatures have included U.S. Pat. No. 5,121,292 to David F. Bell et. al., where liquid nitrogen was used to chill a cold head. In turn, the cold head was engaged with the cold plate of a cryo cooled chamber containing individual chips attached to a substrate. In turn, the substrate is in contact with the heat transfer plate of the cryo cooled unit. While this structure is quite effective to cool individual chips, it does not lend itself to the cooling of memory cubes each containing a plurality of chips, and especially when several memory cubes may be required within a single cryo cooled unit.